In an image forming apparatus, a semiconductive roller produced by forming a semiconductive rubber composition into a roller form and crosslinking the rubber composition is generally used as a charging roller for uniformly electrically charging a surface of a photoreceptor body, as a developing roller for developing an electrostatic latent image formed by light-exposing the electrically charged photoreceptor surface into a toner image, as a transfer roller for transferring the formed toner image onto a paper sheet or the like, or as a cleaning roller for removing toner from the photoreceptor surface after the transfer of the toner image onto the paper sheet or the like.
The semiconductive roller is incorporated in the image forming apparatus with a shaft such as of a metal inserted through and fixed to a center through-hole thereof for use as the charging roller or the like.
In general, the rubber composition to be used as a material for the semiconductive roller is imparted with ion conductivity by blending an ion-conductive rubber (e.g., an epichlorohydrin rubber) as a rubber component.
Further, a diene rubber is generally used in combination with the ion-conductive rubber as the rubber component in order to improve the mechanical strength and the durability of the semiconductive roller and to impart the semiconductive roller with rubber characteristic properties, i.e., to make the semiconductive roller flexible and less susceptible to permanent compressive deformation with a reduced compression set.
Further, a coating film such as of a urethane resin is generally formed on an outer peripheral surface of the semiconductive roller to be brought into contact with the surface of the photoreceptor body.
The outer peripheral surface of the semiconductive roller is coated with the coating film for the following reason. When the semiconductive roller is used as the charging roller or the like in direct contact with the surface of the photoreceptor body, the image formation is prevented from being influenced by contamination of the photoreceptor body with substances bleeding or blooming on the outer peripheral surface from the inside of the semiconductive roller.
Further, minute particles of external additives such as silica and titanium oxide externally added to the toner for improvement of the fluidity and the electrical chargeability of the toner, or minute particles occurring when toner particles are finely broken during repeated image formation, for example, are prevented from adhering to the outer peripheral surface of the semiconductive roller and gradually accumulating on the outer peripheral surface to influence the image formation.
The coating film is generally formed by applying a coating liquid onto the outer peripheral surface of the semiconductive roller by a coating process such as a spraying method or a dipping method, and then drying the coating liquid. Therefore, the coating film is liable to suffer from contamination with dust and other foreign matter, uneven thickness and other defects during the coating process.
Particularly, if the semiconductive roller suffering from any of these defects is used as the charging roller, it is impossible to uniformly electrically charge the surface of the photoreceptor body. Problematically, this may result in defective image formation such as uneven image density.
In addition, the coating film formation technique, which is an established technique, has little room for improvement. Therefore, it is difficult to significantly reduce the incidence of the defects (defect percentage) as compared with the current technique. This may reduce the yield and the productivity of the semiconductive roller, thereby increasing the production costs.
To cope with this, a thin oxide film is formed instead of the coating film in the outer peripheral surface of the semiconductive roller.
After the semiconductive roller is formed from the semiconductive rubber composition containing the diene rubber as the rubber component (by the molding and the crosslinking of the semiconductive rubber composition), the oxide film is formed in the outer peripheral surface of the semiconductive roller through an oxidization reaction of the diene rubber exposed on the outer peripheral surface by irradiating the outer peripheral surface with ultraviolet radiation in an oxidizing atmosphere or by exposing the outer peripheral surface in an ozone atmosphere (see, for example, Patent Document 1).
Therefore, the oxide film is free from contamination with foreign matter during the formation thereof. In addition, the oxidation reaction uniformly proceeds in the outer peripheral surface of the semiconductive roller, thereby suppressing variations in the thickness of the oxide film.
Particularly, where the semiconductive roller having the oxide film is used as the charging roller, it is possible to uniformly electrically charge the surface of the photoreceptor body, thereby preventing the defective image formation such as image density unevenness.